The present invention relates to a measuring sensor, in particular for determining the oxygen content in the exhaust gases of internal combustion engines, having a sensor element which is axially arranged in a metallic housing and which is contacted by at least one connecting cable, which is brought axially out of the housing, through a support sleeve member, and which has a cable insulation, a reference atmosphere for the sensor element being introducible into the housing, and the cable insulation, at the surface area, having, at least in certain regions, at least one gas-permeable section, such that the reference atmosphere arrives inside the cable insulation and, from there, in the housing.
A measuring sensor or a xcex-probe is known from German Patent No. 196 31 501. In the known measuring sensor, the connecting cable insulation is at least somewhat porous over a certain section. The openings placed in the connecting cable insulation are covered by an additional porous PTFE film, to protect them from external contamination. This forms a continuous air-exchange channel between the measuring sensor and the external air. This design approach is relatively economical, since it does not necessitate any additional outlay with respect to the number of components used. However, adhering precisely to a defined porosity is difficult.
From other xcex-probes, one knows of design approaches where Gore-Tex (porous PTFE film, which is permeable to air, but is impermeable to water and oil, and is fabricated using a special process of the firm Gore) is used directly at the cable output of the measuring sensor. These known design approaches require a relatively substantial outlay for design integration, but hardly provide protection from contamination caused by street dirt, oil, undersea coatings, inter alia, and, moreover, they limit the maximum thermal loading capacity at the cable output of the probe.
An object of the present invention is to provide a measuring sensor, in particular a measuring sensor suited for use as a xcex-probe, having a xe2x80x9cventilation sectionxe2x80x9d situated at a relatively cold location of the measuring sensor, and which will make it possible to minimize changes in porosity caused by temperature variations and external contamination.
By providing for the gas-permeable region of the cable insulation to be directly contiguous to the cable-output end of the support-sleeve member and to be sheathed there by a porous tube of gas-permeable PTFE material, while allowing for a radial clearance between the porous PTFE tube and the gas-permeable section of the cable insulation, the present invention not only ensures an optimal ingress of air, i.e. ingress of reference atmosphere to the inside the cable insulation, but also facilitates integration of the porous tube of gas-permeable PTFE material, using relatively simple means, at this relatively cold end of the measuring sensor.
To ensure the radial clearance between the porous PTFE tube and the cable insulation, an inner collet (clamp-type sleeve) is situated between the cable insulation and the porous PTFE tube, and surrounds the cable insulation with radial clearance. This collet has at least one transverse bore in the gas-permeable section of the cable insulation, thereby ensuring free admission of air to the gas-permeable section of the cable insulation.
At this point, the porous PTFE tube is preferably attached to the support-sleeve member and to the inner collet by providing an outer collet surrounding the support-sleeve member and the inner collet with rounded, tamped-out sections at an axial distance, to form a keyed connection (using form locking) between the porous PTFE tube and the cable-output end of the support-sleeve member. These rounded, tamped-out sections of the outer collet define a radial bulge of a specific length, ensuring a free supply of air to a defined length of the porous PTFE tube. In the bulge region, the outer collet has at least one transverse bore which is able to be aligned with the transverse bore in the inner collet. A preformed PTFE tube which protects the cable from breakage and the influences of temperature can be simply integrated at the probe outlet in that the tamped-out section at the cable-output end of the outer collet wedges the preformed PTFE tube radially between the porous PTFE tube and the inner collet.
To provide further protection from external contamination, the xe2x80x9cventilated sectionxe2x80x9d, i.e. the bulge of the outer collet, is able to be sheathed by a silicon-coated, woven-glass tube, which covers the transverse bore or transverse bores in the outer collet, protecting it/them from external contamination. This likewise minimizes any change in the porosity properties of the ventilated section over the service life of the measuring sensor.
Gore-Tex, which, as already mentioned, is fabricated from a porous PTFE film, using a special process of the firm Gore, is preferably used for the porous PTFE tube, as it is permeable to air, but is impermeable to water and oil. One can select the appropriate Gore-Tex quality to adapt the breathability to the requirements of the measuring sensor, within a broad range and with minimal divergence. Thus, for example, contaminant residues resulting from the manufacturing process are able to be easily eliminated using porous membranes, a smaller porosity being advantageous in the case of a more serious risk of contamination, for example from gasoline, engine cleaners, and the like. The inner and outer collets can preferably be made of metal, in particular of stainless steel, and the support-sleeve member of silicon material.